The present invention relates to liquid crystal display, more specifically, to a liquid crystal panel and a liquid crystal display device using a parallel electric field mode, and to a liquid crystal driving method using the parallel electric field mode.
Liquid crystal display devices are widely used in monitors for mobile phones, pagers, notebook computers, desktop personal computers and workstations as display devices capable of high-resolution color display. Heretofore, a simple matrix driving method and an active matrix driving method have been proposed as methods of driving the above-mentioned liquid crystal display devices for performing color display. Among these driving methods, the active matrix driving method utilizing thin-film transistors is widely adopted because of capabilities to constitute small and flat display devices and to offer high image quality.
Meanwhile, a wide viewing angle characteristic which effectuates visual identification of liquid crystal display from various angles is also required in liquid crystal display devices. As similar to other characteristics, such viewing angle enhancement is also recognized as an important problem for liquid crystal display devices.
In order to achieve the above-described viewing angle enhancement, conventionally disclosed is a liquid crystal display device using a pair of comb electrodes as a driving method to set a direction of electric fields to be applied to liquid crystal as approximately parallel to a substrate surface (such a method is hereinafter referred to as a “parallel electric field mode driving method”). FIG. 15 shows a schematic exploded perspective view of a liquid crystal panel for driving liquid crystal by a parallel electric field mode driving method conventionally known as an in-plane switching (IPS) method (M. Oh-e, M. Ohta, S. Aratani, K. Kondo, Asia Display '95, 577(1995) and M. Ohta, M. Oh-e, K. Kondo, Asia Display '95, 707 (1995)).
The liquid crystal panel using the conventional parallel electric field mode driving method shown in FIG. 15 drives liquid crystal molecules by holding a liquid crystal cell 74 between a polarizer 70 and an analyzer 72. In FIG. 15, for the convenience of explanation, the liquid crystal cell 74 is illustrated as exploded. The liquid crystal cell 74 includes transparent substrates such as glass substrates 76a and 76b and liquid crystal molecules 78 sealed between the glass substrates 76a and 76b. Each of the glass substrates 76a and 76b is provided with an alignment film 80 subjected to an alignment process in a given direction by a method such as rubbing, in order to align and fix the liquid crystal molecules 78 severally in a proper direction. Conventionally, films such as polyimide films or carbon films containing DLC are used as the alignment films 80.
In the liquid crystal cell 74 in accordance with the conventional parallel electric field mode driving method as shown in FIG. 15, unillustrated comb-shaped electrodes are formed on one of the glass substrates 76a constituting the liquid crystal cell 74 on the side facing the liquid crystal molecules, by a mode such as sputtering or vacuum deposition. As electric fields are formed between mutually opposing teeth of the comb electrodes, the liquid crystal molecules 78 are driven and liquid crystal display is thereby effectuated.
FIG. 16 is a perspective view showing a detailed constitution of a comb electrode 82 usable in the liquid crystal panel shown in FIG. 15. The comb electrode 82 shown in FIG. 16 is formed on the glass substrate 76a and has a constitution including two arrays of wiring 82a and 82b severally connected to terminals of mutually different electric potential such that the electric fields are formed between the mutually opposing teeth. The comb electrodes 82 shown in FIG. 16 are formed over a display portion of the glass substrate 76a, whereby a necessary display area is secured. The comb electrodes 82 are insulated from the liquid crystal molecules 78 with an insulative film such as the alignment film 80 which is indicated by broken lines. Accordingly, the comb electrode 82 applies electric fields necessary for driving to the liquid crystal molecules 78.
FIG. 17 is a view illustrating movement of the liquid crystal and the direction of the electric fields when the electric fields are applied from the comb electrode 82 to the liquid crystal panel shown in FIG. 15. When a voltage is applied to the comb electrode 82, then as shown in FIG. 17, an electric field as illustrated by an arrow B is formed between the opposing teeth 82a and 82b of the comb electrode 82. The liquid crystal molecules 78 change alignment thereof in accordance with the electric field B. Then the liquid crystal panel allows a direction of polarization of light A irradiated from background illuminating means such as a backlight unit to rotate in accordance with the direction of the liquid crystal molecules 78 so that the light A passes through the analyzer 72, thus effectuating display. In FIG. 17, a state of polarization of the light which passed through the liquid crystal cell 74 is illustrated by A′.
The above-described parallel electric field mode driving method possesses a favorable characteristic that brightness of the display portion of the liquid crystal panel varies largely with respect to viewing angles. By utilizing the large viewing angle characteristic, the parallel electric field mode driving method is particularly used in a wide viewing angle liquid crystal display device.
In the meantime, a vertical electric field driving method refers to a driving method to apply electric fields to liquid crystal molecules in a perpendicular direction with respect to a substrate surface. The vertical electric field driving method has been used from the beginning of researches and developments of liquid crystal displays. Enhancement of viewing angles for a liquid crystal panel has been also studied on the above-described vertical electric field driving mode. Among those studies, an OCB mode proposed by Miyashita et al (SID '95 Digest: p. 797, 1995) and a HAN mode proposed by Saitoh et al (Asia Display '95: p. 589, 1995) severally realize high-speed response. Accordingly, those modes are expected to be applied to usage such as a liquid crystal display device for movie display.
However, in an active matrix liquid crystal display device of the IPS method with a fine viewing angle characteristic, it is necessary to set a cell gap of a liquid crystal cell to 5 (micrometers) or less for realizing high-speed response, or considerably narrower than 5 (micrometers) in order to meet more practical high-speed response. In addition to such inconvenience, it is necessary to set a driving voltage upon driving as high as 10 V or higher. Meanwhile, although the above-mentioned OCB mode driving method has a fine viewing angle characteristic and requires a proper driving voltage, it is necessary to form “bent” alignment of liquid crystal molecules at an initial state. Accordingly, the OCB mode driving method has inconvenience that it is difficult to form such a “bent” state stably. Meanwhile, the HAN mode driving method is an innovative method which shows a high viewing angle and high speed response characteristic, and capable of low voltage driving as well. Nevertheless, the HAN mode is not practicable because the mode has a serious defect to generate a large residual voltage and tends to cause image sticking. In the following, description will be made in detail regarding the HAN mode driving method by use of FIG. 18.
FIG. 18 is a schematic cross-sectional view of a liquid crystal cell used in the HAN mode driving methods. As shown in FIG. 18, the conventional HAN mode driving liquid crystal cell includes a pixel electrode 86 formed on a glass substrate 84, and a common electrode 90 formed on a glass substrate 88. The respective electrodes 86 and 90 are insulated from a composition containing liquid crystal molecules by alignment films 92a and 92b. Moreover, the alignment films 92a and 92b are composed so as to have mutually different aligning modes, whereby liquid crystal display is effectuated by generating alignment variation of the liquid crystal between perpendicular alignment and horizontal alignment.
However, the above-described constitutions of the alignment films in the HAN mode driving method cause an electrically asymmetric interface characteristic inside the cell. Such an asymmetric interface characteristic is assumed to be a factor to generate the above-mentioned image sticking (Tanaka et al, Jpn. J. Appl. Phys. Vol. 38, L1115 (1999)).